In the field of drilling in the exploration for oil and gas, an important component is that of the formulation of drilling muds. Drilling muds are the fluids which are used to maintain pressure, cool drill bits and lift cuttings from the holes and vary in composition over a wide spectrum. Generally, drilling muds are based on aqueous formulations or oil-based formulations.
A conventional oil-based drilling mud formulation is comprised of basically the following ingredients: oil (generally No. 2 diesel fuel), emulsifying agents (alkaline soaps of fatty acids), wetting agents (dodecylbenzene sulfonate), water, barite or barium sulfate, (weighting agent), asbestos (employed as viscosification agent), amine treated clays (also as viscosification agent).
The above combination of ingredients is generally formulated to possess various weights based primarily on amount of barite added. For example, a typical drilling mud can vary in specific gravity from a range of about 7 lbs. per gallon up to 17 lbs. per gallon or even greater. This variation in specific gravity is primarily controlled by the amount of barite added. The above formulations perform adequately in a number of applications, primarily those where the use of oil-based drilling muds is dictated by the lack of stability of the formation in which drilling is taking place. For example, in various types of shale formation, the use of conventional water-based muds can result in a deterioration and collapse of the shale formation. The use of the oil-based formulations circumvents this problem. However, it is observed that the current oil-based drilling muds have some significant disadvantages. One disadvantage is that the incorporation of asbestos or asbestos fines can incur significant health problems both during the mud formulation and potentially during the subsequent use of such formulations. Therefore, it is desirable to eliminate the use of asbestos completely in such drilling muds. On the other hand, the use of substitutes for asbestos in this application, heretofore, has not been particularly successful in that the resulting viscosification agents must maintain adequate viscosities under the drilling conditions which can involve high temperature and high shear conditions.
This invention describes an approach to viscosification of oil-based drilling muds which permits the substitution of sulfonated ionomers for asbestos fines. The resulting polymer-modified drilling muds display viscosities which are in a desirable range for drilling mud applications, good viscosities at retention after aging at temperatures as high as 325.degree. F. for 24 hours.
The types of sulfonated polymers that are envisioned in the present invention are sulfonated ethylene/propylene/5-phenyl-2-norbornene terpolymers. Ionomers of EPDM's have, in the past, been prepared by sulfonating terpolymers of ethylene-propylene and a diolefin. The resulting polymers contain aliphatic sulfonates after "neutralization". Many of the sulfonates of commerce are aromatic sulfonates. The aromatic sulfonates are generally preferred because they are prepared more cleanly and are more stable than olefin (aliphatic) sulfonates. For example, long chain olefins are utilized to alklate aromatics like benzene, and the alkylated aromatics are sulfonated to produce detergents. Even though such a process requires an extra step, it is generally preferred, rather than to sulfonate the olefin directly. Thus, detergents produced via the aromatic sulfonation are produced more cleanly and have greater stability.
By analogy, we have produced ethylene/propylene-based ionomers in which the sulfonate group is bonded to an aromatic ring. The resulting polymers are thermally stable and are useful viscosifiers for drilling muds. These polymers have better low temperature properties than sulfonated polystyrenes while maintaining the stability associated with aromatic sulfonates. These polymers possess suitable solubilities in the drilling mud environment.